The present invention relates to resonance type power converter units and more particularly to a resonance type power converter unit employable in a lighting apparatus for illumination using a high resonance frequency or a resonance type active filter for suppression of higher harmonics.
In recent years, a discharge tube (for example, fluorescent lamp) of the type in which a DC voltage is converted into a high-frequency AC voltage by means of a lighting circuit using an inverter and the high-frequency AC voltage is applied to a resonance load circuit inclusive of the discharge tube has been used widely. The resonance load circuit includes a resonance inductor and a resonance capacitor which are used for setting a resonance frequency. This type of lighting circuit is an inverter circuit having two power semiconductor switching devices which are connected to form a half bridge configuration between positive and negative polarities of a DC power source, and it applies the high-frequency AC voltage across the resonance load circuit. Current flowing through the resonance means can be controlled by changing the operating frequency of the inverter. On the assumption that the switching frequency for alternately turning on and off the two power semiconductor devices is f and the resonance frequency determined by the resonance inductor and capacitor is fo, lamp current changes to make luminescence of the discharge tube unstable unless f is constant for fo.
As a first conventional example for stabilization of the driving frequency of the switching devices, a drive unit as disclosed in JP-A-8-37092 is known. The conventional drive unit features in that it has 1) a timer circuit for generation of a square wave of a desired frequency, 2) high-side and low-side drive circuits adapted to drive two power semiconductor switching devices of an inverter in accordance with a drive signal from the timer circuit, 3) high-side and low-side dead time delay circuits adapted to prevent simultaneous conduction of the two power semiconductor switching devices and 4) a level shift circuit adapted to convert a signal based on a reference of low-side common potential into a signal based on a reference of high-side common potential in order for the drive signal from the timer circuit to be transmitted to the high side, and that these circuits are built in a single integrated circuit. It will be appreciated that in the above conventional example, the switching devices are driven in accordance with the frequency of the timer circuit and essentially, this frequency is asynchronous with a resonance current of the lamp. As a second conventional example for performing switching synchronous with a resonance current flowing through the lamp, a control circuit disclosed in JP-A-8-9655 is known. In the control circuit, a terminal voltage across a circulating diode (having the function not to prevent backward current) provided for a power semiconductor switching device is detected to turn on the power semiconductor switching device, current of the power semiconductor switching device is integrated by means of an integrator, and the device is turned off when the integrated value exceeds a reference value.
In the aforementioned first conventional example, elements in each of the timer circuit, level shift circuit and dead time delay circuit have irregularities in characteristics or suffer from a temperature rise and expectantly, the oscillation frequency will shift and the operation will delay. Especially, in an electrodeless lamp reported recently, luminescence of a discharge tube is controlled by a method in which the frequency is raised to a value of several of MHz so that a high-frequency magnetic field may be generated by a high-frequency AC current and plasma in the lamp tube is sustained by the magnetic field. In such a resonance type inverter of several of MHz as above, a shift of oscillation frequency and a delay in operation cannot be negligible. More particularly, on the assumption that the delay time is 0.1 .mu.s, the operation delay is only 0.5% of one wavelength in a stabilizer using an ordinary driving frequency of 50 kHz but amounts up to 20% of one wavelength in an electrodeless lamp of 2 MHz. Thus, when the high-frequency resonance type inverter of several of MHz is controlled by the conventional method, there arises a problem that the irregularity in driving frequency due to the operation delay becomes considerably large.
In the case of the lighting apparatus, the switching frequency f of the inverter is related to the resonance frequency fo determined by the resonance inductor and resonance capacitor as shown in FIG. 2 and in relation to an operational boundary at a resonance point (f=fo), a lagging phase is defined for f&gt;fo and a leading phase is defined for f&lt;fo. Current IL of the resonance circuit has a smaller value for either the lagging phase or the leading phase than that at the resonance point. Accordingly, the inverter is desired to be operated near the resonance point but in the case of the leading phase, there arises a problem that a through-current flows in the inverter.
When the inverter has power semiconductor switching devices Q1 and Q2 which incorporate circulating diodes QD1 and QD2, respectively, and an output voltage Vo is taken out of a connecting node of the devices Q1 and Q2, the leading phase is defined as a state in which a waveform of the resonance current IL leads a waveform of the output voltage Vo by a phase .phi.. In the case of the leading phase, one-cycle operation proceeds in such a manner that during on-duration of the device Q1, the resonance current IL is switched from positive to negative and the current flows through the circulating diode QD1. Subsequently, the control circuit turns off the switching device Q1 and conversely turns on the device Q2, a backward voltage is abruptly applied to the diode QD1 through which the forward current has passed till then. As a result, electrons and positive holes stored in the diode QD1 (hereinafter referred to as residual carriers) are exhausted and a backward current (hereinafter referred to as a backward recovery current) from cathode to anode flows through the diode QD1. This current passes through the device Q2, behaving as a through-current for the inverter. The time required for the residual carriers to be exhausted is described as backward recovery time of diode in specifications of the device and even for a device of short backward recovery time called a high-speed diode, the backward recovery time is 0.05 to 0.1 .mu.s. When the resonance type inverter of several of MHz is operated near the resonance point, the possibility that the leading phase due to the irregularity in switching frequency occurs is high and besides a high frequency prevails. For these reasons, a loss due to the through-current behaves as a factor which determines a thermal, operational limit of the lighting apparatus.
In the second conventional example, the power switching device is driven by detecting a voltage of a diode connected in parallel with the switching device and therefore, operation at the lagging phase is warranted. As described hereinafter, a larger current is required upon start of the discharge tube than during lighting and as the resonance current flowing through the discharge tube increases, the time required for the aforementioned integral value to reach the reference value decreases, so that the driving frequency of the switching device increases. On the other hand, in the case of lagging phase, the resonance current has such a characteristic that as the driving frequency of the switching device increases, the resonance current decreases as shown in FIG. 2. This leads to a problem that a large current cannot be obtained upon start.